This invention relates to a precision chain drive of the type in which a roller chain is trained around and tensioned between a pair of rotatable wheels and is advanced around a racetrack-shaped path when one of the wheels is driven. The driven wheel is in the form of a sprocket wheel while the other wheel is an idler. A chain of this type sometimes is driven intermittently and carries fixtures which dwell at successive work stations when the chain stops.
Precision roller chains which have an even number of links inherently present a problem when an attempt is made to tension the chain between a sprocket wheel and an idler wheel and to keep the two straight runs of the chain parallel to one another. That is, the chain tends to become tight when the links are in one angular position relative to the wheels and then tends to loosen when the links advance to a different angular position. The cyclic tightening the loosening inherently occurs as the links approach and recede away from the points at which the runs of the chain become tangent to the wheels.
To provide a precision chain drive, compensation must be made for the cyclic tightening and loosening of the chain. Without compensation, the chain, when in its naturally tightest position, will break if the spacing between the two wheels is sufficiently great to keep the chain tight when the chain is in its naturally loosest position. Conversely, if the spacing between the wheels is sufficiently small to prevent breakage of the chain when the chain is in its naturally tightest position, the chain will be loose and imprecise when in its naturally loosest position.
One way of attempting to compensate for the cyclic tightening and loosening of the chain is to guide the rollers of the chain such that the rollers move along an angled path as the rollers approach and recede from the wheels. This approach, however, still does not enable the chain to be extremely tight and, perhaps even more detrimentally, causes the rollers to impact against the guide means and the wheels. This results in noise and abrupt shock loads and particularly when heavy fixtures are carried by the chain.
A second compensation method involves the use of a wheel which is spring loaded so that it can automatically move toward and away from the other wheel and change the effective center distance between the wheels to accommodate the cyclic tightening and loosening of the chain. This method, however, also is unsatisfactory from the standpoint that it requires repetitive movement of the mass of the spring loaded wheel and produces noise and shock loads.